Power-plant excitation system



Jan. 23, 1951 c. ATwELL ErAL, 2,539,173

POWER PLANT EXCITATION SYSTEM.

Filed Nov. 20, 1947 i V-fl Curve for full prime-mover a outpuf x1002 Jpeed Genrmfar ulafim I at 10025 eed I n I I '3 i 3 {2 ,4: fizapeed N J 0 lk I G E 0 REM. HMPERES F? 1. F7 a.

lga dj jumjzbn a toad .fazuraziam at .90; speed A! se aggzasz load .Safurafian at Z lowest new #0 speed 0 gas-turbine 7" 7' M 0 FIELD EXCITHUON 0 HELD EXCITHUON WITNESSES: INVENTORS Clarence A7. flzwell and M Charlrg s 1-. Jenkins.

ATTORNEY Patented Jan. 23, 1951 POWER-PLANT EXCITATION SYSTEM Clarence A. Atwell, Pittsburgh, and Charles F.

Jenkins, Laughiintown, Pa, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 20, 1947, Serial No. 787,205

7 Claims. 1

Our invention relates to an excitation-system for a direct-current generator which is used for loading a gas turbine, as in a turbine-electric locomotive.

An object of our invention is to provide a power-plant employing a prime-mover, such as a gas turbine, which varies its power-output in proportion to about the sixth power of its speed, or at least faster than the fourth power of its speed, in combination with dynamo-electric generator-means which is mechanically coupled to said prime-mover and is adapted to absorb substantially all of the power of the prime-mover,

and which is provided with an excitation-system operation, control, and manufacture, hereinafter described and claimed, and illustrated in the accompanying drawing, wherein:

Fig. l is an output-speed curve for a gas turbine in comparison with a Diesel or gasoline engine;

Fig. 2 is a set of volt-ampere curves illustrative of our invention;

Fig. 3 is a simplified symbolic diagram of circuits and apparatus illustrative of a preferred form of embodiment of our invention; and

Figs. 4 and 5 are excitation-diagrams illustrative of our invention.

The art of exciting a direct-current generator so as to absorb the power-output of a Diesel or gasoline engine is well developed. When a gas turbine is used for the prime-mover, however, a different situation exists. This is shown in Fig. 1, wherein the curve OP represents a typical shape of the horse-power-speed curve (H. P.- R. P. M.) for a Diesel or gasoline engine, whereas the curve GP represents a typical shape of the H. P.-R. P. M. curve for a gas turbine. It will be noted that the output-curve OP, for a Diesel or gasoline engine, starts at the origin, and swings up, with increasing speeds, until it is fairly flat in the full-speed range of the engine. In the case of a gas turbine, however, no poweroutput is produced until the turbine reaches a certain speed G, and from that speed up to the iulloperating speed, the change in the poweroutput is very rapid, being approximately as the sixth power of the speed, in the operating range of the turbine.

' The significance of this speed-sensitive powerchange, in relation to the volt-ampere requirements of an electric generator which constitutes practically the sole load on the turbine, is illustrated in Fig. 2, wherein the curve 6 shows the volt-ampere curve for an ideal generator which is developing a constant output, as its current and voltage change, keeping this constant output at the power-output of the gas turbine when it is operating at 100% speed. Assuming that the turbine-output varies exactly as the sixth power of its speed, then at speed the power-output of the turbine will be reduced to 73.5% of its full output which is shown in curve 6, this reduced output being indicated by the curve I in Fig. 2.

A generator which loads a gas turbine, therefore, must be controlled so as to be very sensitive to speed-changes, so that, when the generator is subjected to an overload, or to a demand for more power, it will not deliver that overload, but on the contrary, will drop its load, as its speed falls off, more rapidly than the turbine drops its power-output, in response to a falling off of speed. Otherwise, the generator would stall the prime-mover or gas turbine, whenever there was a momentary increased-load demand on the generator.

In accordance with our present invention, assuming a sixth-power speed-response in the turbine-output, the generator-voltage, at any given number of load-amperes, will be reduced at a rate better than the sixth power of speed, say the six and one-half power of speed. If such a speed-sensitive voltage-control is used on the generator, it is not particularly essential that the generator have a volt-ampere characteristic which approximates the shape of the constantpower curves 8 and I in Fig. 2, as the system will operate whether this characteristic volt-ampere shape is closely approached or not, provided that the volt-ampere characteristic of the generator is a drooping characteristic. A typical voltampere characteristic, showing the generatorregulation at speed, is indicated by the curve 8 in Fig. 2. It will be noted that this curve 8 crosses the turbine-output curve 6 at two points 8 and 8", so that a generator, with sucha characteristic, will have a stable operating-condition as indicated by the upper point 8, when absorbing the power of a prime-mover which is represented by the volt-ampere curve 6.

If, now, the generator-voltage can be made so 3 sensitive to the speed of the generator that it varies as, say, the six and. one-half power of the speed, a drop in speed, to a value equal to 95% of its full speed, will cause the generatorvoltage to drop to 71.6% of its former value, at each value of the load-current of the generator, as indicated by the curve 9 in Fig. 2. It will be noted that this new generatorcharacteristic curve 9 intercepts the corresponding turbineoutput curve 1 at two points 9' and 9", so that the generator will have a stable operatingcondition at the upper intersection 9, which was seen to correspond to -a sightly increased ampere-load on the generator, and a very much decreased output-voltage.

The turbine-generator combination is thus seen to be stable, inasmuch asthe generator can not stall the turbine, and the generator will adjust its output-voltage and load-current soas to absorb the power delivered by the turbine under 'u'n'der aliopcrating conditions. Thi powercan regulated bya suitable throttle-cntro1on-the gas turbine. Such a power-plant system is thus seeri' -to -beadmirably suited for the requirements 'of apower-plantfor an electric-l'occmofiive having'traction-motors energized from such a gen- 'erator.

id-Fig. 3, a gas turbine is symbolically indi- 'cat'ed, at H; as having a shaft l2 which is di *re'ct-connected to a generator G which is -illustrated 'as having-an armature-circuit connected to the output-leads Li and L2, serially including a differential series field-winding D, and wdmrnutating-winding C. "The field-member "o f -t he generator G is-also provided with a main field win'ding' FI and a smaller auxiliary field- 'W inding F2, nearly all of the field of the generatorbeing supplied by the winding Fl.

"The field-excitation system which is shown in Fig/'3 comprises a main exciter ME and two pil'ot-exciters PEl and PEZ, all three exciters being"mo'unted "on-a common shaft l3 which is driven at an convenient speed, proportional to the speed of the'prime-moveror gas turbine I l,

as byin'eansof a pulley-drive Hi from the tur- ,1

bine=shaft I 2.

"*Ea'chbfthe 'exciters ME, PEI, andPEZ is a diie'ct cu'rrent generator or dyna1no=electric machinewhich has an output-voltage which varies according to a certain power or thes'peed. lhe maim'excit'er ME has a principal field winding F3 which supplies most of the field of the "exciter, and an auxiliary field 'winding F t. armatur'e-circuit of the main exciter ME is used to energize them'ain field-winding Flof the generator' 'G. The main field-winding of the main 'exciter ME is self-excited from the armature circuit of the main exciter, as shown in Fig. 3. 'By the term self-excited, we mean to-"ref'er to a condition in which the field-exciting ampere-turns of a direct-current generator are obtained from a field-winding which is connected in-shunt across the armature circuit of that generator,-as shown for the main field-winding F3 of the main exciter ME in Fig. 3.

1 Except for its size, the first pilot-exciter PEI is similar to. the main exciter ME. This first pilot-exciter has a self-excited main field-winding F5 and a separately excited auxiliaryfieldwinding F6, and the armature-circuitpf this first pilot-exciter PEI energizes the auxiliary winding F4 of the main exciter ME.

The second pilot exciter PER has a" self-excited=field winding F1, and its armature-circuit 4;. is used to energize the auxiliary field-winding F6 of the first pilot-exciter PEI.

All seven of the field-windings Fl to F! of the excitation-system shown in Fig. 3 are provided with serially connected, adjustable, field-rheostats R1 to R7, respectively, whereb the fieldstrengths may be adjusted as needed.

The auxiliary field-Winding FZ 0f the generator G is adjustably energized, through its rheostat R2, from a constant-voltage source which is represented by a battery B, so that the voltagelevel of the volt-ampere characteristic curve of the generator G, such as the curve 8 shown in Fig. 2, can be raised or lowered to control the operating-point 8 (Fig. 2) at which the generator operates; at any speed of the prime-mover or gas turbine I I.

It will be noted that the exciters ME and PEI aremaihly self-excited, but each exciter obtains a small portion of its excitation from another exciter, the voltage ofwhich is sensitive to -the speed of the excitel shaft i3, andhence to :the speed-*ofthe turbine-shaft -12. The reason for this combination of self-excitation and-speedresponsive separateex citation is to obtaine. much greater voltage-variation, in response-to speed, than would bepossible without the'sel fi-excifation. This is explained in Figw lywhereinl the curve OX is the full-speed load-saturation curve of either oneof the eX-citers ME or PE-l, and' OY is the load-saturation curve at %--speed. If the total field-excitation of the-excitenat fhll speed, is shown-by the distance OM,--th'e=excita tion-lineMX determines the operating-voitage -X at 'fullspeed. If this excitation QM -were-enltirely produced by means oraseparate-excitation source which varies its voltage in exact proportion to thespeed, then a -10% --speed-reduetion would result in a 10% -excitation-reduction R,

"excitation OT will represent the part (if tile -total excitation OMwhich is supplied-by *the separate excitation, and the excitation TM will represent the-part or the total 'exeitation=-'whicli is supplied by self-excitation. If, now, the -sspa-rateexcit"ationpart OT is reduced 10%; as *indieated at Q, corresponding to a 10% spe'ed red u ction, --on the assumption of a separate exeitatiei-isource which variesdts voltage-directly asthe speed, thenan excitation-line T2 is "obtained, substantially parallel to 'TX, and intersecting the load sejturation curve' OY atZ, for' thespeed 'condition, thus determining the output veltage- 'Z-of the exciter. It will be*n'oted that a 1%)%-specd-rtdtiction at -first resulted in chan ging the excitervoltage ironfX to Yg-assum an of the field is produced--by separate exci tatien;whereas, in'the case of an-excier having a partial-selcf em citation, a 10% speed-red-ub'ti-on results-in*a larger voltage 'drop, from X tcrZ, which islarger than the voua enmprmmx to Y.

"The final exciter, PE2;*'is=made-entirelyselfexcited, becauseit is thefllast'nne in the-series, and does not receive excitati'on from a*preceding exciter. This'exciter shouldhave a steep's'atur'ation-curve,-as indicated at' *OHkin Fig. 5, for the speedyso as to-produce" a rapid-"voltage drop in response -to' speed. By prcperly "setting the external field-resistor R1 of this exciter, the self-excitation line OSI can be set, as shown in Fig. 5, so that it is approximately tangent to the beginning of the load-saturation curve ON at the lowest power-output speed 0G (Fig. 1) of the gas turbine, or the speed at which the gas turbine starts to produce power. The system would then not build up any voltage below this turbine-speed 0G (Fig. 1), and therefore would not absorb any electrical power below this speed. At 90% of full speed, the load-saturation curve OK, in Fig. 5, determines the exciter-voltage K at the intersection with the self-excitation line OSI.

The second pilot exciter PE2 can be built, in accordance with the principles just described, so as to have a terminal-voltage which varies as much as the one and one-half power of its speed, with any given setting of its field-rheostat R1. Each of the other two exciters PEI and ME can be built, in accordance with the described principles of design, so as to have a terminal-voltage which varies as much as the square of the speed. The voltage of the generator G varies practically in proportion to its speed, for any given strength of its field-excitation. Hence, the voltage-variation of the generator, with the above-described excitation-system, is in accordance with about the six and one-half power (1 /2+2+2+1) of the speed, so that the generator G drops its voltage, and hence its load, faster than the gas turbine drops its power-output, in response to a drop in speed.

The difierential field D of the generator is not essential, as it is merely a means for augmenting the effect of armature-reaction in controlling the droop of the volt-ampere characteristic of the generator, in a known manner.

While we have illustrated. our invention in a single preferred form of embodiment, in which a single generator G is utilized to absorb all of the power of the prime-mover H, and in which three dynamo-electric exciter-machines are used, we wish it to be understood that these precise details are not essential, as various changes may be made in regard to the number of machines used, or the substitution of one machine for another, or the substitution of other equivalents, and also various equipments may be added to or subtracted from the illustrated embodiment without departing from the essential principles of our invention. We desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language.

We claim as our invention:

l. A power-plant comprising a prime-mover of a type having a power-output which varies faster than the fourth power of its speed, dynamoelectric generator-means mechanically coupled to said prime-mover and adapted to absorb substantially all of the power of said prime-mover, a load-circuit for the generator-means, and an excitation-system which inherently varies the exciting-voltage of said generator-means so as to make the generator-output vary in response to its speed at a rate faster than the output-variation of the prime-mover, said excitation-system being characterized as follows: said generatormeans having a main exciting-winding means for providing its principal excitation, an auxiliary excitation-winding means, an adjustable constant-voltage energizing-means for the auxiliary excitation-winding means of the generatormeans, a main dynamo-electric exciter-means mechanically coupled to said prime-mover and electrically connected to energize said main exciting-winding means of the generator-means, said main exciter-means having a self-excited main exciting-winding means for *providin its principal excitation, and an auxiliary excitationmeans, and one or more dynamo-electric pilotexciter means mechanically coupled to said prime-mover and electrically connected so as to energize the auxiliary excitation-means of the main exciter-means, said pilot-exciter means having a self-excited exciting-winding means for providing its principal excitation.

2. A power-plant comprising a prime-mover of a type having a power-output which varies approximately as the sixth power of its speed, dynamo-electric generator-means mechanically coupled to said prime-mover and adapted to absorb substantially all of the power of said primemover, a load-circuit for the generator-means, and an excitation-system which inherently varies the exciting-voltage of said generator-means so as to make the generator-output vary in response to its speed at a rate faster than the outputvariation of the prime-mover, said excitationsystem being characterized as follows: said enerator-means having a main exciting-winding means for providing its principal excitation, an auxiliary excitation-winding means, an adjustable constant-voltage energizing-means for the auxiliary excitation-Winding means of the generator-means, a main dynamo-electric exciter means mechanically coupled to said prime-mover and electrically connected to energize said main exciting-winding means of the generator-means, said main exciter-means having a self-excited main exciting-winding means for providin its principal excitation, and an auxiliary excitationmeans, a first dynamo-electric pilot-exciter means mechanically coupled to said prime-mover and electrically connected to energize the auxiliary excitation-means of the main exciter-"neans, said first pilot-exciter means having a self-excited main exciting-winding means for providing its principal excitation, and an auxiliary excitationmeans, and a second dynamo-electric pilot-exciter means mechanicall coupled to said primemover and electrically connected to energize the auxiliary excitation-means of the first pilot-exciter means, said second pilot-exciter means having a self-excited exciting-winding means for providing itsprincipal excitation.

3. A power-plant comprising a gas turbine, dynamo-electric generator-means mechanically coupled to the gasturbine and adapted to absorb-substantially all of the power of said gas turbine, a load-circuit for the generator means, and an excitation-system which inherently varies the exciting-voltage of said generator-means so as to make the generator-output vary in response to its speed at a rate faster than the outputvariation of the gas turbine, said excitation-system being characterized as follows: said generator-means having a main exciting-winding means for providing its principal excitation, an auxiliary excitation-winding means, an adjustable constant-voltage energizing-means for the auxiliary excitation-winding means of the generator-means, a main dynamo-electric excitermeans mechanically coupled to said gasturbine and electrically connected to energize said main exciting-winding means of the generator-means, said main exciter-means having a self-excited main exciting-winding means for providing its principal excitation, and an auxiliary excitationmeans, a first dynamo-electric pilot-exciter means mechanically coupled to said gas turbine means and electrically "connected .to a'energize;v the auxiliaryxexcltation means of 'the main excitermeans, said. first pilot-exciterv means :havingta self-.excit'ed .main excitingewinding Lmeans .for providing aitsnprincipal excitation, and an .iauxiliary excitation-means, and a second dynamoelectric:pilot-exciter means mechanically coupled to said gas Lturbineland electrically connectedto energize the auxiliary excitationemeansziof ,lthe first'pilot exciterimeans, said second. pilot-exciter means having .ia ;q=self-excited zexcitingewinding meansior providing itszprincipal excitation.

4. ,ncplurality ofr direct-current dynamo-electric .machines, ,a driving ImeansUfor; drixringpsaid machinesat: corresponding::sp.eeds Whichare subje'ct to some variation under; different zloadeccnditions, :iaz first one i of .said :machines .ihavingwa selfe'excited'zmain lexcitingawinding :means "for providing. its principal excitation,:randzxani'auxiliaryexcitatiommeans, and azsecond cnergcf; said machines being electricallyxcconnectedJtozzeriergizetthe auxiliary excitation-means CfiSaidLfiY-St machine, said second. machine :having ;:ai3lf excited excitingfiwinding :means for providing its principal excitation.

15. A plurality .of direct-current dynamoeelectric machines, ,a driving-means for ;,driving., said machinesat corresponding speeds whicharezsubject -.to some variation under :different loadaconditions a .first one-of said machineshavingza selfeexcited main exciting-winding means ,zfor providing its ,principaliexcitation, and .an sallX- iliaryexcitationemeansga second one. of saidmachines being electrically..connected *to 1 energize the auxiliary, excitation-means 1 of said first imachine,:saidsecond machine ahaningaaselfgexcitied main. exciting-winding means; for: providing s-its principal excitation, and anzauxiliary excitationmeans; and va thirdone: of ;said,;,machines;:being electrically connected.- .to energize. ltheiauxiliary excitation means .ofrsaid second machine, said third :.machine :havingia .SfilfnBXGitfid excitingw-indingemeanslfor providingrits principalv :excitaztion.

1' 6. In 'combinationg a dynamo-electric;generator-meansshaving-.azmain exciting ewinding means for providing its principal "excitation, wand ;an auxiliary iexciting -winding :ameans, .a :drivingmeans hfOl driving said generator-:means lat-,a speed which is subject 1170,. somenvariation-under different loadeconditionaa loadacircuit for the generator-means, an adjustable constant-voltage energizingemeansnior the "auxiliary lex'citationew'inding .meansof the generaton-imeanaa main idynamoeelectric .eXciter-rneans mechanically 1 coupled "to *saidgdriving-meanswand; ,electrically connected to energize ;said-rmain exciting-winding means of the generator-means, saidrmain .exciter-means having a self-excited main :excitingewinding-nneans forsrproviding its principal excitation, and amauxiliaryexcitationmeanspand 1 one-.101: :more. dynamo-electric pilotexciter means mechanicall rvcoupled :to saididriving-means and electrically connected so asrto; energiz theauxiliary:excitationemeans ofzxtheimain exciter -means,-: said .pilot-exciter 1 means having.:.a. self eexcited exciting-Minding means for providing its principal excitation.

1' 7 .g In combinationpsa dynamo-electric .generator -meanshaving: axmainexcitingewindingmeans for providing its principal excitatiomlandianauxiliaryi exciting-windingmeans,::;a ldrivingemeans for ,dzzivingsaidrgenerator meansat aispeed which is :subject to :some variation. under difierent:lcadconditions, are load-circuit for :the generatormeansyan Iadjustablev constantevoltage energizing-means' Jfor :theeauxiliaryr-xexcitaticngwinding means ofithle ;generator-means;ia=mainxiynamcelectric exciten-m'ean's mechanically coupled to said :drivingemeans :and electrically; connectedrto energize A said main :excitingewinding ;.meansunf the generatoremeans, fsaid" Lmain exciteremeans having :a :selfeexcited main =.-exciting-winding means for providing its principal ,excitatiomand an: auxiliary excitation-'meanaaaufirstedynamoelectric pilot-exciter means mechanically-coupled to said drivingemeans-and electricallyficonnected to energizeithe. auxiliar iexcitation meansgof. the main exciter meanssaid :first-piloteexcitersmeans having "a :.1se1f'-'excited;-main exciting-winding means for providing its principal excitationgand an auxiliary excitation-means, and axsecondidyname-electric pilot-exciter. means; mechanically coupled to. :sa'idv idrivingemeansruiancl selectifically connected to energize the auxiliaryziexcitationmeans of the first piloteexcitermeans,saidrsecnnd pilot-exciterimeans having aselfeexcitedexditingwinding imeans for :.-providing. iits-principah excitation.

CLARENCE A. ATWsELL. .F. ;Y-JENKINS.

qREFERENEES :GITED UNITEDMS'DATES. PATENTS 

